Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation to a commercial market is not feasible, produced natural gas is often processed into LNG for transport to market. The LNG is typically transported via specially built tanker ships, and then stored and revaporized at an import terminal near the market. The equipment used to liquefy, transport, store, and revaporize natural gas is generally quite expensive; and a typical conventional LNG project can cost from $5 billion to $10 billion, including field development costs. A typical "grass roots" LNG project requires a minimum natural gas resource of about 280 Gm.sup.3 (10 TCF (trillion cubic feet)) and the LNG customers are generally large utilities. Often, natural gas resources discovered in remote areas are smaller than 280 Gm.sup.3 (10 TCF). Even for natural gas resource bases that meet the 280 Gm.sup.3 (10 TCF) minimum, very long-term commitments of 20 years or more from all involved, i.e., the LNG supplier, the LNG shipper, and the large utility LNG customer, are required to economically process, store, and transport the natural gas as LNG. Where potential LNG customers have an alternative source of gas, such as pipeline gas, the conventional LNG chain of delivery is often not economically competitive.
A conventional LNG plant produces LNG at temperatures of about -162.degree. C. (-260.degree. F.) and at atmospheric pressure. A typical natural gas stream enters a conventional LNG plant at pressures from about 4830 kPa (700 psia) to about 7600 kPa (1100 psia) and temperatures from about 21.degree. C. (70.degree. F.) to about 38.degree. C. (100.degree. F.). Up to about 350,000 refrigeration horsepower are needed to reduce the temperature of the natural gas to the very low outlet temperature of about -162.degree. C. (-260.degree. F.) in a conventional two-train LNG plant. Water, carbon dioxide, sulfur-containing compounds, such as hydrogen sulfide, other acid gases, n-pentane and heavier hydrocarbons, including benzene, must be substantially removed from the natural gas during conventional LNG processing, down to parts-per-million (ppm) levels, or these compounds will freeze, causing plugging problems in the process equipment. In a conventional LNG plant, gas treating equipment is required to remove the carbon dioxide and acid gases. The gas treating equipment typically uses a chemical and/or physical solvent regenerative process and requires a significant capital investment. Also, the operating expenses are high in relation to those for other equipment in the plant. Dry bed dehydrators, such as molecular sieves, are required to remove the water vapor. The scrub column and fractionation equipment are used to remove the hydrocarbons that tend to cause plugging problems. Mercury is also removed in a conventional LNG plant since it can cause failures in equipment constructed of aluminum. In addition, a large portion of the nitrogen that may be present in natural gas is removed after processing since nitrogen will not remain in the liquid phase during transport of conventional LNG and having nitrogen vapors in LNG containers at the point of delivery is undesirable.
Containers, piping, and other equipment used in a conventional LNG plant are typically constructed, at least in part, from aluminum or nickel-containing steel (e.g., 9 wt % nickel), to provide the necessary fracture toughness at the extremely cold processing temperatures. Expensive materials with good fracture toughness at low temperatures, including aluminum and commercial nickel-containing steel (e.g., 9 wt % nickel), are typically used to contain the LNG in the LNG ships and at the import terminals, in addition to their use in the conventional plant.
A typical conventional LNG ship utilizes large spherical containers, known as Moss spheres, to store the LNG during transport. These ships currently cost more than about $230 million each. A typical conventional project to produce LNG in the Middle East and transport it to the Far East might require 7 to 8 of these ships for a total cost of about $1.6 billion to $2.0 billion.
As can be determined from the above discussion, the need exists for a more economical system for processing, storing, and transporting LNG to commercial markets to allow remote natural gas resources to compete more effectively with alternative energy supplies. Furthermore, a system is needed to commercialize smaller remote natural gas resources that would otherwise be uneconomical to develop. In addition, a more economical gasification and distribution system is needed so that LNG can be made economically attractive to smaller consumers.
Consequently, the primary objects of the present invention are to provide a more economical system for processing, storing, and transporting LNG from remote sources to commercial markets and to substantially reduce the threshold size of both the reserve and the market required to make an LNG project economically feasible. One way to accomplish these objects would be to process the LNG at higher pressures and temperatures than is done in a conventional LNG plant, i.e., at pressures higher than atmospheric pressure and temperatures higher than -162.degree. C. (-260.degree. F.). While the general concept of processing, storing, and transporting LNG at increased pressures and temperatures has been discussed in industry publications, these publications generally discuss constructing transportation containers from nickel-containing steel (e.g., 9 wt % nickel) or aluminum, both of which may meet design requirements but are very expensive materials. For example, at pp. 162-164 of his book NATURAL GAS BY SEA The Development of a New Technology, published by Witherby & Co. Ltd., first edition 1979, second edition 1993, Roger Ffooks discusses the conversion of the Liberty ship Sigalpha to carry either MLG (medium condition liquefied gas) at 1380 kPa (200 psig) and -115.degree. C. (-175.degree. F.), or CNG (compressed natural gas) processed at 7935 kPa (1150 psig) and -60.degree. C. (-75.degree. F.). Mr. Ffooks indicates that although technically proven, neither of the two concepts found `buyers`--largely due to the high cost of storage. According to a paper on the subject referenced by Mr. Ffooks, for CNG service, i.e., at -60.degree. C. (-75.degree. F.), the design target was a low alloy, weldable, quenched and tempered steel with good strength (760 MPa (110 ksi)) and good fracture toughness at operating conditions. (See "A new process for the transportation of natural gas" by R. J. Broeker, International LNG Conference, Chicago, 1968.) This paper also indicates that an aluminum alloy was the lowest cost alloy for MLG service, i.e., at the much lower temperature of -115.degree. C. (-175.degree. F.). Also, Mr. Ffooks discusses, at p. 164, the Ocean Phoenix Transport design, working at a much lower pressure of about 414 kPa (60 psig), with tanks that could be constructed of 9 percent nickel steel or aluminum alloy; and indicates that, again, the concept did not appear to offer sufficient technical or financial advantages to become commercialized. See also: (i) U.S. Pat. No. 3,298,805, which discusses the use of a 9% nickel content steel or a high strength aluminum alloy for making containers for the transport of a compressed natural gas; and (ii) U.S. Pat. No. 4,182,254, which discusses tanks of 9% nickel or similar steel for the transport of LNG at temperatures from -100.degree. C. (-148.degree. F.) to -140.degree. C. (-220.degree. F.) and pressures of 4 to 10 atmospheres (i.e., of 407 kPa (59 psia) to 1014 kPa (147 psia)); (iii) U.S. Pat. No. 3,232,725, which discusses transportation of a natural gas in a dense phase single-fluid state at a temperature as low as -62.degree. C. (-80.degree. F.), or in some cases -68.degree. C. (-90.degree. F.), and at pressures at least 345 kPa (50 psi) above the boiling point pressure of the gas at operating temperatures, using containers constructed from materials such as 1 to 2 percent nickel steel which has been quenched and tempered to secure an ultimate tensile strength approaching 120,000 psi; and (iv) "Marine Transportation of LNG at Intermediate Temperature", CME March 1979, by C. P. Bennett, which discusses a case study of transport of LNG at a pressure of 3.1 MPa (450 psi) and a temperature of -100.degree. C. (-140.degree. F.) using a storage tank constructed from a 9% Ni steel or a 31/2 % Ni quenched and tempered steel and having 91/2 inch thick walls.
Although these concepts are discussed in industry publications, to our knowledge, LNG is not currently commercially processed, stored, and transported at pressures substantially higher than atmospheric pressure and temperatures substantially higher than -162.degree. C. (-260.degree. F.). This is likely due to the fact that an economical system for processing, storing, transporting, and distributing LNG at such pressures and temperatures, both via sea and via land, has not heretofore been made commercially available.
Nickel-containing steels conventionally used for cryogenic temperature structural applications, e.g., steels with nickel contents of greater than about 3 wt %, have low DBTTs (a measure of toughness, as defined herein), but also have relatively low tensile strengths. Typically, commercially available 3.5 wt % Ni, 5.5 wt % Ni, and 9 wt % Ni steels have DBTTs of about -100.degree. C. (-150.degree. F.), -155.degree. C. (-250.degree. F.), and -175.degree. C. (-280.degree. F.), respectively, and tensile strengths of up to about 485 MPa (70 ksi), 620 MPa (90 ksi), and 830 MPa (120 ksi), respectively. In order to achieve these combinations of strength and toughness, these steels generally undergo costly processing, e.g., double annealing treatment. In the case of cryogenic temperature applications, industry currently uses these commercial nickel-containing steels because of their good toughness at low temperatures, but must design around their relatively low tensile strengths. The designs generally require excessive steel thicknesses for load-bearing, cryogenic temperature applications. Thus, use of these nickel-containing steels in load-bearing, cryogenic temperature applications tends to be expensive due to the high cost of the steel combined with the steel thicknesses required.
Five co-pending U.S. provisional patent applications (the "PLNG Pat. No. Applications"), each entitled "Improved System for Processing, Storing, and Transporting Liquefied Natural Gas", describe containers and tanker ships for storage and marine transportation of pressurized liquefied natural gas (PLNG) at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature in the broad range of about -123.degree. C. (-190.degree. F.) to about -62.degree. C. (-80.degree. F.). The most recent of said PLNG Patent Applications has a priority date of May 14, 1998 and is identified by the applicants as Docket No. 97006P4 and by the U.S. Patent and Trademark Office ("USPTO") as application Ser. No. 60/085467. The first of said PLNG Patent Applications has a priority date of Jun. 20, 1997 and is identified by the USPTO as application Ser. No. 60/050280. The second of said PLNG Patent Applications has a priority date of Jul. 28, 1997 and is identified by the USPTO as application Ser. No. 60/053966. The third of said PLNG Patent Applications has a priority date of Dec. 19, 1997 and is identified by the USPTO as application Ser. No. 60/068226. The fourth of said PLNG Patent Applications has a priority date of Mar. 30, 1998 and is identified by the USPTO as application Ser. No. 60/079904. However, the PLNG Patent Applications do not describe systems for vehicular, land-based distribution of PLNG. As used herein, "vehicular, land-based distribution of PLNG" means distribution of PLNG from central processing or storage facilities to end-user or storage facilities primarily over land, such as by truck, railcar, or barge through existing road, railroad, and land-locked water systems.
LNG is routinely distributed from central processing or storage facilities to end-user sites by truck, railcar, or barge through existing road, railroad, and land-locked water systems. Other cryogenic fluids, such as liquid oxygen, liquid hydrogen, and liquid helium are also routinely distributed by these means. The market for LNG, in particular, has grown in recent years because of the clean-burning characteristics of natural gas. To meet this increasing market demand, delivery of produced natural gas in the form of PLNG, as compared to LNG, can be beneficial to the end-user because the PLNG is more economically processed, provided that an economical means for transporting and delivering the PLNG is made available. Additionally, as compared to CNG, the higher liquid density of PLNG translates into higher product mass or energy for a given volume.
Carbon steels that are commonly used in construction of commercially available containers for fluids do not have adequate fracture toughness at cryogenic temperatures, i.e., temperatures lower than about -40.degree. C. (-40.degree. F.). Other materials with better cryogenic temperature fracture toughness than carbon steel, e.g., commercial nickel-containing steels (31/2 wt % Ni to 9 wt % Ni) with tensile strengths up to about 830 MPa (120 ksi), aluminum (A1-5083 or A1-5085), or stainless steel are traditionally used to construct commercially available containers that are subject to cryogenic temperature conditions. Also, specialty materials such as titanium alloys and special epoxy-impregnated woven fiberglass composites are sometimes used. However, containers constructed from these materials often lack adequate strength at traditional wall thicknesses, e.g., about 2.5 cm (1 inch), to contain pressurized, cryogenic temperature fluids, so wall thicknesses of such containers must be increased to add strength. This adds weight to the containers that must be supported and transported, often at significant added cost to a project. Additionally, these materials tend to be more expensive than standard carbon steels. The added cost for support and transport of the thick-walled containers combined with the increased cost of the material for construction can often make projects economically unattractive. These disadvantages make currently commercially available materials economically unattractive for constructing containers and systems for vehicular, land-based distribution of PLNG. The discovery of containers suitable for marine transport of PLNG, as discussed in the PLNG Patent Applications, combined with current capabilities for processing PLNG, make eminent the need for systems for economically attractive vehicular, land-based distribution of PLNG. A significant portion of vehicular, land-based distribution cost is the capital cost associated with vehicle container design and fabrication. A significant cost reduction in the vehicle container cost would ultimately translate into an overall improvement in the economics of vehicular, land-based transportation of PLNG, as well as that of LNG and other cryogenic fluids.
The availability of a more cost-effective source of natural gas transported and distributed in the form of a liquid would provide a significant advancement in the ability to utilize natural gas as a fuel source. The following are brief descriptions of existing and emerging applications that use natural gas for energy and that would benefit significantly from the availability of a more economical system for transportation and distribution of natural gas in the form of PLNG.
LNG is routinely trucked to meet fuel needs at remote sites where the infrastructure for natural gas distribution does not exist. Additionally, local conditions are increasingly making transported LNG a competitive economic alternative to gas pipelines for several major energy projects. An Alaskan gas company has proposed a $200 Million project for remote LNG baseload systems in seventeen communities in southeastern Alaska. The company also expects to truck LNG 300 miles from a liquefaction plant on Cook Inlet to Fairbanks starting in November, 1997. In eastern Arizona, a recent feasibility study has shown that remote baseload LNG supply facilities may offer an attractive lower-cost energy solution to a number of isolated communities without current access to gas pipelines. In addition to trucks and barges, railcars may also be used to transport LNG. These represent new trends in large-volume LNG transportation and usage with potential for substantial growth. The emerging PLNG technology could make economically feasible the use of PLNG as fuel in these and other similar land-based applications, if a more economical means of vehicular, land-based distribution of PLNG, were available.
Secondly, trucking LNG to meet fuel needs of certain manufacturing plants has also become a competitive economical alternative. The most recent example is a company in Hampton, N.H., which switched from a gas-supply contract with propane as a back-up to the exclusive use of LNG to run a 4,000 horsepower engine for electric power generation and to operate two process boilers on vaporized LNG. Again, further improvements in distribution costs would likely result in an increased number of similar applications.
Further, there is an increasing growth in the use of `portable pipeline`--transportable LNG/vaporizer--systems to maintain continuous uninterrupted gas supply. This is to help gas companies avoid service interruption and to continue the flow of natural gas to customers during peak demand periods, such as cold winter days, emergency from a damaged underground pipe, maintenance on a gas system, etc. Depending on the particular application, an LNG vaporizer may be installed or located at a strategic spot on the natural gas distribution system, and when operating conditions warrant, LNG tanker trucks are brought in to provide the LNG that is vaporized. Currently, to our knowledge, there are no commercial tanker trucks for transporting PLNG, instead of LNG, to such a vaporizer for providing additional gas during peak demands.
Finally, there are projections that several of the current and future major LNG importers in Asia offer the most potential for LNG use as vehicle fuel (as much as 20% of imports). Trucking of LNG to the refueling stations may be the most attractive economic option depending on local conditions. In particular, in the absence of an existing infrastructure for gas distribution, cost-effective tanker design may make PLNG, distribution (by truck, railcar, or barge through existing road, rail, and land-locked water systems) a more attractive and economic alternative.
A need exists for economical systems for vehicular, land-based distribution of PLNG to allow remote natural gas resources to compete more effectively with alternative energy supplies. Additionally, a need exists for more economical systems for vehicular, land-based distribution of LNG and other cryogenic fluids. As used hereinafter, the term "tanker truck" is meant to include any means for vehicular, land-based distribution of PLNG, LNG, or other cryogenic fluids, including without limitation, tanker trailers, railcars, and barges.
Therefore, a particular object of the present invention is to provide economical systems for vehicular, land-based distribution of LNG at substantially increased pressures and temperatures over conventional LNG systems. Another object of the present invention is to provide such systems having storage containers and other components that are constructed from materials having adequate strength and fracture toughness to contain said pressurized liquefied natural gas.